[PPT] - Using Classes Effectively [Andersen, Gries, Lee, Marschner, Van PowerPoint Presentation

SLIDE 1

Using Classes Effectively

Lecture 18

CS 1110:

Introduction to Computing Using Python

[Andersen, Gries, Lee, Marschner, Van Loan, White]

SLIDE 2

Announcements

A3 due tonight at 11:59pm.
Spring break next week:
No office hours
No consulting hours
Limited piazza

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Making Arguments Optional

We can assign default values to

init arguments

Write as assignments to

parameters in definition

Parameters with default values

are optional

Examples:
p = Point3() # (0,0,0)
p = Point3(1,2,3)

# (1,2,3)

p = Point3(1,2) # (1,2,0)
p = Point3(y=3) # (0,3,0)
p = Point3(1,z=2) # (1,0,2)

class Point3(object): """Instances are points in 3d space x: x coord [float] y: y coord [float] z: z coord [float] """ def __init__(self,x=0,y=0,z=0): """Initializer: makes a new Point Precondition: x,y,z are numbers""" self.x = x self.y = y self.z = z …

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SLIDE 4

Making Arguments Optional

We can assign default values to

init arguments

Write as assignments to

parameters in definition

Parameters with default values

are optional

Examples:
p = Point3() # (0,0,0)
p = Point3(1,2,3)

# (1,2,3)

p = Point3(1,2) # (1,2,0)
p = Point3(y=3) # (0,3,0)
p = Point3(1,z=2) # (1,0,2)

class Point3(object): """Instances are points in 3d space x: x coord [float] y: y coord [float] z: z coord [float] """ def __init__(self,x=0,y=0,z=0): """Initializer: makes a new Point Precondition: x,y,z are numbers""" self.x = x self.y = y self.z = z …

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Assigns in order Use parameter name when out of order Can mix two approaches

Using Classes Effectively

SLIDE 5

Making Arguments Optional

We can assign default values to

init arguments

Write as assignments to

parameters in definition

Parameters with default values

are optional

Examples:
p = Point3() # (0,0,0)
p = Point3(1,2,3)

# (1,2,3)

p = Point3(1,2) # (1,2,0)
p = Point3(y=3) # (0,3,0)
p = Point3(1,z=2) # (1,0,2)

class Point3(object): """Instances are points in 3d space x: x coord [float] y: y coord [float] z: z coord [float] """ def __init__(self,x=0,y=0,z=0): """Initializer: makes a new Point Precondition: x,y,z are numbers""" self.x = x self.y = y self.z = z …

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Assigns in order Use parameter name when out of order Can mix two approaches

Using Classes Effectively

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On Tuesday, we learned how to make:

Class definitions
Class specifications
Class variables
Methods
Attributes (using self)
A constructor with __init__

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Today

Class definitions
Class specifications
Class variables
Methods
Attributes (using self)
A constructor with __init__

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Type: set of values and the operations on them
int: (set: integers; ops: +, –, *, /, …)
Time (set: times of day; ops: time span, before/after, …)
Rectangle (set: all axis-aligned rectangles in 2D;
ps: contains, intersect, …)
To define a class, think of a real type you want to make

Designing Types

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Making a Class into a Type

1. Think about what values you want in the set
What are the attributes? What values can they have?
2. Think about what operations you want
This often influences the previous question
To make (1) precise: write a class invariant
Statement we promise to keep true after every method call
To make (2) precise: write method specifications
Statement of what method does/what it expects (preconditions)
Write your code to make these statements true!

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Planning out a Class: Time

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What attributes?
What invariants?
What methods?
What constructor?

(24-hour clock)

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Planning out a Class

class Time(object): """Instances represent times of day. Instance Attributes: hour: hour of day [int in 0..23] min: minute of hour [int in 0..59]""" def __init__(self, hour, min): """The time hour:min. Pre: hour in 0..23; min in 0..59""" def increment(self, hours, mins): """Move this time <hours> hours and <mins> minutes into the future. Pre: hours is int >= 0; mins in 0..59""" def isPM(self): """Returns: this time is noon or later."""

Class Invariant

States what attributes are present and what values they can have. A statement that will always be true of any Time instance.

Method Specification

States what the method does. Gives preconditions stating what is assumed true of the arguments.

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Implementing a Class

All that remains is to fill in the methods. (All?!)
When implementing methods:
1. Assume preconditions are true
2. Assume class invariant is true to start
3. Ensure method specification is fulfilled
4. Ensure class invariant is true when done
Later, when using the class:
When calling methods, ensure preconditions are true
If attributes are altered, ensure class invariant is true

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